Now you hear it, now you don’t! Bad weather sometimes brings out the worst in a radio communications system–or maybe it only seems that way. Keeping up-to-date records of equipment modifications and repairs help to troubleshoot rainy-day problems.
When it rains, it pours. Not just a logo on the salt box, mind you, but radio problems for the local county sheriff. It’s one of those days when the rain just keeps coming down. We are all in the shop trying to keep dry when the trouble is first noticed. Ruben, the chief technician in this three-tech shop, is monitoring our most important, not to mention largest customer, the Sheriff’s Patrol. He sensed trouble before the telephone rang. I’ll bet you have days just like this one in your shop. “Radio shop, this is Ruben, how can I help you?” “Ruben, we got trouble,” the sheriff’s watch commander said. “With the storm, we are ‘up to our ears in alligators’ with traffic and street flooding problems, and the radio is not working right.” “I noticed on my monitor that some of your mobiles are cutting out.” answered Ruben, “Is that what you mean?” “10-4 to that, Ruben. I have 12 units out on the storm patrol, and some of them keep cutting out at dispatch, but they all seem to hear the dispatcher OK.” “We’ll get right on it, sir,” replied Ruben.
This is an unexpected problem, particulary because the shop recently had performed routine maintenance on the sheriff’s entire system. The Sheriff’s Patrol uses multiple-site simulcast transmitters and voting receivers–pretty standard for covering wide areas in law enforcement.
“Well, boss,” Ruben said, “we better put the staff on the go-plan and get ’em moving. The sheriff needs help now.” “Ruben,” I asked, “what do you think–is it a vehicle or a system problem?” “Couldn’t say,” answered Ruben, “but if we get someone to monitor the voter panel, maybe we can find out quickly. “Hey Jerry, get over to the sheriff’s equipment vault pronto and monitor the voter panel. The sheriff’s in trouble and we need to troubleshoot the system.” “OK, I’m on my way,” answered Jerry, heading out the door, umbrella, raincoat and test gear in hand.
It is only one block to the sheriff’s equipment vault, and Jerry was there in a flash, watching the traffic on the voter. It didn’t take him very long to see the nature of the beast. The watch commander’s trouble was not coming from the mobiles, but from the system.
“Well, boss,” telephoned Jerry, “tell Ruben the problem occurs when any mobiles or portables hit receiver site three. It doesn’t matter which mobile is talking; when it is voted to site three it goes mute, and they lose it. I ‘scoped the input to the voter, and the receiver stays muted and the vote tone is at the correct minus 20 dB’s.”
“Ok, Jer,” I replied. “I’ll send Karen to site three, and you stay there to do an end-to-end test. Keep the watch commander advised if you take any equipment out of service, OK?” “Gotcha,” said Jerry as he hung up.
Karen was not long getting to site three, including the preparations for a rainy day. The system is composed of Ericsson General Electric Mastr II UHF radio units at each site. The normal preventive maintenance is logged, and the site log is where Karen started looking. She thought it strange that the unit was just P.M.’ed, and now it was failing. The log showed that the unit had failed the CTCSS receive sensitivity test. This unit’s P.M. test required 850Hz to 1,000Hz deviation to open the receiver on a test tone. A normal module requires a deviation level of about 250Hz to 300Hz. The entire module was replaced with one of the new units from Ericsson. The new design has dip-switch programming, where the old type used a Versatone frequency setting chip in a socket. The programmable design minimizes the spare parts stock problems, and perhaps equally important, the technician does not find arrive at site “A” with only a tone chip for site “B” in the parts box. The shop has been replacing the old-style units when they go bad. We also are finding the bench repair of the units is more costly than the replacement assembly.
At site three, Karen found that the old unit had been replaced with a l9D432500G3. The G3, or group 3, is for decode-only and, operating as a duplex, four-wire audio radio, this site uses a decode-only and no-CTCSS on transmit. “Hello,” called Karen on the service phone, “are you there, Jer?” “Contact,” answered Jerry from the sheriff’s main equipment room, “Whatcha need, kiddo?” “Call the watch commander and get us permission to take site three out for a while so you and I can take a look at it, will ya?” “No problem,” answered Jerry, “I have him on the phone now, and we’re clear for a 30-minute outage.” “OK, now will you disable the voter for this site and monitor the incoming audio on a test speaker?” “Voter off, speaker on!” “I’m setting up the ‘IFR’ and I’ll give you some test tone. Here it is; what do you see?” “Hey, hey, hey, Karen,” Jerry replied, “I’m just getting a vote tone, not a test tone. Are you hooked up correctly?” “You bet, Jer,” confirmed Karen. “Whoops! Try again, I can hear the test tone on the service speaker. Can you hear it now?” “OK now!” said Jerry. “Well, I’ll be darned. I got a test tone on the receiver and I hear it from the service speaker, and you get it at the voter too. Hang on while I check some more.” “Cool,” said Jerry, “While you do that, I’ll check the levels on the other channels. Holler when you need me.” “Re-enable the remote transmit line so the dispatcher can still talk off this site, and then don’t get far from the service phone, Jer.” “I won’t. Just holler when you need me.”
Karen couldn’t get a handle on the problem at this point, and went to the instruction book for the Mastr II. While looking through the book, she still had a monitor speaker on the line output. Something caught her ear. This sheriff’s radio system returns all the mobiles’ audio to all the transmitter sites and retransmits just as a repeater would, except it sounded to her like a classic case of desense. Mobiles would cut out, and the system would un-key, and they would be received again for a “click” and disappear again to repeat the cycle. She reached over and killed the remote transmit enable switch on the control shelf. All of a sudden she heard all the mobile traffic on her monitor speaker! No more sounds of mobiles cutting out. “Hey, Jer!” exclaimed Karen, “Something weird is going on.” “Do you still have the voter disabled from my site?” “Sure, why?” “Well, keep your monitor on the incoming line, and then when I give you a test tone, remote-key the transmitter and see what happens to the audio, OK?” “OooKaay . . . It is keyed . . . . Whoowa! streamed Jerry, “When I keyed, the incoming audio from your receiver muted and returned to vote tone. Whatcha doing over there?” “We have a receiver that is muting on transmit, Jer,” said Karen with confidence. Call the shop and let’s get some more info.”
Jerry, Karen and Ruben were soon on a conference call, and Ruben put it on the speaker phone at the shop. Ruben got out the sheriff trouble and maintenance file. Meanwhile, I located the Ericsson General Electric Mastr II update file. At the site Karen referred to the Mastr II Instruction Book, and Jerry was just waiting to be helpful. “Karen, you are in the drivers seat,” I started, “The rain hasn’t stopped, but it is otherwise quiet at the shop. Bring us up to date on the watch commander’s complaint.”
“Here’s what we have determined so far, boss,” began Karen, “The problem is certainly a system problem, and it is centered at site three. Furthermore, we have tracked it to a strange problem at the radio.” “Strange, how?” I interjected. “Strange in that a radio suddenly starts to mute it’s receiver when it transmits,” she said, “and to me that is strange because muting is a fixed setup choice, and how can it change?” “Yeah,” added Jerry. “What else have you found?” I asked. “The log shows that Jerry P.M.’d this site last week,” began Karen, “and the Channel Guard decoder was out of spec. He replaced it with a new one and returned the site to service as normal. No other trouble found. Right Jer?” “Right as rain,” chimed in Jerry, “The site worked fine when I left it last week. I followed the checklist and logged it all.” “Bet you didn’t try the receiver with the transmitter keyed,” Karen interrupted, “because that is not in our normal procedure. As I think about it, boss, I wonder if it has to do with the replacement Channel Guard module?” “You are getting warm, Karen,” I replied while looking over our notes in the E.G.E. Mastr II file, “Jerry, is the system still disabled?” “Is now.” “OK, now Karen, you take the Channel Guard module out of the receiver so you can look at it, OK?” I requested. “Got it . . . Am I looking for something, and is it on the front or back?” she replied. “Foil and solder side,” I told her, “where the connections leave edge jack J-908, pin 6 and go to hybrid U1002, pin 8; look to see if that trace is cut. Pin 6 is six in from the corner, with the corner starting as number 1.” “Nope, no cut anywhere,” mused Karen, “Not there, not anywhere. Why are we looking for a cut?” “Well, for now, take your tool kit scribe and cut through that trace,” I instructed, “and replace the card and re-test with the system.” “Got it. Hey Jeeerrry,” requested Karen, “Monitor for a test tone and then remote key the transmitter, OK?” “Voter off,” murmured Jerry, “monitor on, here we go . . . keyed . . . Eureka!” “I think he’s saying it works,” Karen added. “Well, ‘another fine fix you’ve gotten us into’ Karen,” I said. “You both forgot the staff meeting we had about two years ago,” Ruben added. “The replacement decoders didn’t work in a four-wire duplex system, because they mute the receiver line when they see a PTT on the bus. The PTT bus has to be cut on the decoder board, or, what just happened to our customer will happen every time!
“Karen, close the site; Jerry, call the watch commander and tell him the system is repaired and back in service,” Ruben instructed. “Jerry,” I said, “Stop and get everyone donuts on the way back. Even better, you and Karen could bring donuts for a week. And pay attention during staff meetings.” “And cows could fly,” replied Karen. “Yeah,” added Jerry, “and remember, we fixed it! How about you getting the treats?” “Ok, you pick ’em up and I’ll buy,” I relinquished. “You guys did a great job on a rainy day. My treat!”